1. Field of the Invention
The present invention relates to optically active compounds and liquid crystal compositions comprising these compounds. The compounds and compositions of the present invention show a ferroelectric liquid crystal phase, and accordingly are useful as electrooptic switching elements such as liquid crystal display devices or the like, and can be used in liquid crystal optical modulators.
2. Related Art Statement
Liquid crystal display devices have various excellent features such as low-voltage operability, low power consumption, being thin and light-weight, being a non-emissive type and easy on the eye, etc. Accordingly, they are in wide use as various display devices.
Liquid crystal display devices using a nematic liquid crystal operating in the so-called twisted nematic mode (TN mode) are in use currently. However, display devices using this kind of nematic liquid crystals have the drawback of being very slow in response as compared to luminescent type display devices such as CRT, EL and the like. Consequently, when such display devices are applied in a display device, particularly a large scale display device capable of displaying a large amount of information, it is impossible to obtain a display of good contrast. Thus the liquid crystal display devices using a nematic liquid crystal have only a very limited applicability. There has recently been developed a liquid crystal display device using a nematic liquid crystal operating in the so-called super twisted nematic mode (STN mode) or SBE and capable of giving a display of improved contrast. Even in this STN mode liquid crystal display device, however, the response is not sufficient, and therefore said device finds also only a very limited application, because it cannot be used for displays capable of displaying a still larger amount of information. Hence, various attempts have been made to develop a new liquid crystal display system giving an excellent response.
Ferroelectric liquid crystals have a memory characteristics and give a high speed response, and accordingly their application to large scale displays is highly expected. As liquid crystals having retroelectric properties, there are known those showing a chiral smectic C phase, a chiral smectic H phase, a chiral smectic J phase, etc. Of these ferroelectric liquid crystals, those showing a chiral smectic C phase are thought to have highest practical utility.
Ferroelectric liquid crystals showing a chiral smectic C phase were first synthesized in 1975 by R. B. Meyer et al.; one typical example thereof is 2-methylbutyl 4-( 4'-n-decyloxybenzylideneamino)cinnamate (hereinafter abbreviated to DOBAMBC) [J. Physique, 36, L69 (197.5)].
A thin film liquid crystal cell was prepared using DOBAMBC and was found to have a high speed response in the order of .mu.s [N. A. Clark et al., Appl. Phys. Lett., 36, 89 (1980)]. Since that time, there was started the development of optical modulation devices (e.g. liquid crystal display devices, photo-printer heads) using a ferroelectric liquid crystal showing a chiral smectic C phase (hereinafter may be referred to simply as "ferroelectric liquid crystal").
As a result, a number of ferroelectric liquid crystal compounds showing a chiral smectic C phase have been developed since then, and various ferroelectric liquid crystal compounds are already known. However, no ferroelectric liquid crystal compound is found yet which has satisfactory reliability and capability for use in display devices, particularly large scale displays, etc.
In order for a ferroelectric liquid crystal to be practically used in a liquid crystal display device, etc., the liquid crystal must be superior in high speed response, orientation, memory characteristics, characteristics of threshold voltage, temperature dependences of these properties, etc. Also, the ferroelectric liquid crystal is required to show a chiral smectic C phase over a wide temperature range so that it can operate within a sufficiently wide temperature range including room temperature, and further to have excellent physical and chemical stabilities.
In order for a ferroelectric liquid crystal to have, in particular, excellent physical and chemical stabilities, good high speed response and good memory characteristics, the liquid crystal must have a large spontaneous polarization.
Among the so far developed ferroelectric liquid crystals, no compound is found yet which satisfies all the above requirements. For example, the above mentioned DOBAMBC, being a liquid crystal of Schiff's base type, is insufficient in chemical stability to water, light, etc., and moreover, has a small spontaneous polarization of 4 nC/cm.sup.2 or below.
Ester type liquid crystals are reported as being ferroelectric liquid crystals which are chemically stable. However, these liquid crystals are not satisfactory because they have no sufficiently large spontaneous polarization and no sufficiently wide temperature range of chiral smectic C phase.
In order to obtain a large spontaneous polarization, there were synthesized compounds having two asymmetric carbon atoms as an optically active group essential for the expression of a chiral smectic C phase.
These compounds include, for example, liquid crystal compounds having a dichiral epoxide side chain [David M. Walba et al., Journal of American Chemical Society, 108, 7424 (1986)], and liquid crystal compounds having a halogen atom and a methyl group on two adjacent asymmetric carbon atoms [cf. e.g. JP-A-168780/1985, 18358/1985, 68449/1986, 40/1987, 46/1987, 103043/1987, 111950/1987, 142131/1987, 175443/1987].
A typical example of the above liquid crystal compounds is ( s)-2-chloro-3-methylbutyl 4'-octylcarbonyloxy-4-biphenylcarboxylate [JP-A-68449/1986 ]. This liquid crystal compound has a very large spontaneous polarization of 180 nC/cm.sup.2 but, being an aliphatic chloro compound, has poor chemical stability. Hence, there was synthesized 4'-octylcarbonyloxy-4-[(s)-2-methoxy-(s)-3-methylpentyloxycarbonyl]bipheny l [JP-A-28036/1987]. This compound has excellent chemical stability but has an insufficient spontaneous polarization of 17 nC/cm.sup.2.